Modern portable telecommunication devices, such as laptop or palmtop computers, personal digital assistants (PDA's), or cellular telephones, are now capable of operating using more than one different communications arrangement, depending upon various external factors. For example, a handheld digital terminal may transmit data to, and receive data from a remote server or host via a wireless local area network (e.g., Wavelan) when the device is indoors. When the device is later moved to an outdoor location, the device may then be arranged to communicate via a cellular (e.g., GSM) network. In the aforementioned example, when the device is moved outdoors, it may transition from a high-bandwidth environment to a low-bandwidth environment, and various other of its communications properties (e.g. bit error rate, delay, jitter, loss, etc.) may also change. These changes typically are evidenced by network or link level protocol signals in the processor that controls the operation of the device.
In prior art portable devices, when an abrupt change or discontinuity, such as a bandwidth reduction, occurs in the communications arrangement, the performance of the device degrades, causing, for example, graphical images to become fuzzy or received signals to become noisy or distorted. In some existing arrangements, this degradation is then sensed, and steps are taken to correct to problem, such as by intentionally permitting a reduction in the precision of the received and/or transmitted signals, decreasing the resolution of the display or reducing the size of the image. Such an approach is reactive, in that the change in the communication arrangement occurs first, and the change in the operation of the portable device occurs later, in response to the detection of the fact that performance has become degraded. In a reactive approach, data can be lost, since the transition itself may take a not insignificant amount of time, and during that time, the device operation is sub-optimum and the quality of service, as perceived by the user, will be degraded.
One example of a previous reactive approach is described generally in a paper entitled "Renegotiable Quality of Service--A New Scheme for Fault Tolerance in Wireless Networks, presented by R-W Chen, P. Krzyzanowski, M. Lyu, C. Sreenan and J. Trotter (all of Lucent Technologies) at the 27.sup.th PTCS, June 1997, Seattle, Wash. This reference relates to a device in which the link layer protocol is arranged to determine that the bandwidth available to the device has changed. The link layer protocol can then signal this change to upper layers of the protocol stack, including, the application layer protocol, responsive to which the application may, for example, change its data rate.
Other reactive approaches have been used in modems, in Internet applications, and in images processed in accordance with MPEG standards. In connection with certain "intelligent" modems, the bit rate at which a modem operates can be changed reactively, in response to channel conditions that are inferred, for example, from a measurement of error rate or constellation dispersion. In connection with the Internet, it is also known in a general way that the TCP/IP protocol stack can be arranged to adapt, over time, to the fact that the available bandwidth available to an Internet device has changed. This adaptation can be used to thereafter cause the application to change its rate, or to at least cause the application's data to be buffered, so that data is not lost in the network due to inadequate bandwidth. With respect to image processing, the MPEG standard contemplates an application protocol layer that can be structured in such a way as to be able to provide outputs at different bit rates, based on being told what bandwidth is available.
In each of the reactive approaches described above, changes occurring in the communications arrangement that a device is using are first observed. Then, changes to the operation of the device itself are brought about by changes in the applications layer protocol. This reactive process takes time, during which data may be lost and/or performance may suffer. Thus, the problem in the prior art is the inability to provide a speedy transition so as to minimize or eliminate data loss or to at least provide a transition that is as unobjectionable as possible, even in the face of some data loss during the transition.